1. Field of the Invention
The invention relates to a weighing machine with a case enclosing the weighing system, provided with an electric or electronic indicator and a weighing scale for holding the material to be weighed, which is movable a limited amount in vertical direction and is supported on the weighting system.
2. Prior Art
Conventional electromechanical weighing machines have an ample electric and electronic system which, to be sure, permits on the one hand a fast and comfortable weighing of the material, but on the other hand threatens to form, by sparkover and shortcircuits, an explosion when the weighing machine is employed in rooms with explosive atmosphere or when explosive material is to be weighed with the weighing machine. In highly sensitive precision weighers the weighing scale for holding the material to be weighed is supported on the weighing system, which is arranged in an enclosing case. The weighing scale support in this structure penetrates the case without contact. The ring slot thus formed is sealed merely by a labyrinth packing, so that the weighing scale support can move vertically, limited by stops, but otherwise passes frictionless through the case.
This renders it impossible to seal the interior of the case, which contains the electric system in a gas-tight manner, so that inside the case the same atmosphere prevails as outside the case.
To be sure, it is principally possible to install electronic component parts in a short-circuit-proof manner, but this is too expensive, considering the ample electric and electronic system of such weighing machines. Such electronic weighing machines could until now not be employed in spaces where an explosion might occur.
Conventional and still used mechanical weighing devices likewise usually have electric component parts, e.g., an illuminating device for the indication. Thus, it is already known (German DE-OS No. 18 00 688) to provide an explosion protection for the illuminating device of a weighing machine, especially for a scale projection device. In this structure, the lamp of the illuminating device is mounted in a protective case arranged in the weigher case and connected to the lighting line via a leakage reactance transformer that reduces the voltage, the primary of said leakage reactance transformer being switched on and off by an explosion protection switch arranged in the weighing case. In this weighing machine, only an illuminating device is used as an electric component part, which in this case may also be provided at a justifiable expense with an explosion protection. As soon as the electric and electronic component parts exceed a certain extent in number and volume, such a measure, as it is proposed by the prior art, is too expensive. Furthermore, the component parts would become too voluminous, so that they could no longer be installed in a standard weighing machine case, e.g., for precision weighers, but would have to be placed beside the weigher. This would restrict the space of operation and impair the weighing comfort.